July 10, 2026 · Vessel Identification
IMO Number Explained: Format, Check Digit and Common Uses
The IMO number is the closest thing a ship has to a lifetime social security number. Once assigned, it never changes — through renamings, re-flaggings, or ownership sales — making it the anchor identifier for port state control, insurance and sanctions checks.
The format
An IMO number is always written as IMO 1234567 — the letters “IMO” followed by exactly seven digits. The last digit is a mathematical check digit derived from the first six.
The check-digit formula
Worked example — IMO 9074729: 9·7 + 0·6 + 7·5 + 4·4 + 7·3 + 2·2 = 63 + 0 + 35 + 16 + 21 + 4 = 139. 139 mod 10 = 9, which matches the seventh digit. Valid.
IMO vs MMSI vs Call Sign
- IMO Number — lifetime hull identifier, 7 digits.
- MMSI — AIS radio identifier, 9 digits, changes on re-flag.
- Call Sign — alphanumeric radio call, assigned by flag state.
Verify any IMO number in one click
Use the Vessel IMO Number Check Digit Validator to confirm a number is mathematically valid and rule out typos or fabricated identifiers before you file paperwork.
Frequently asked questions
What is an IMO number?
An IMO number is a unique 7-digit identifier assigned to a merchant ship's hull by IHS Markit on behalf of the International Maritime Organization. It stays with the vessel for its entire life, regardless of name, flag or owner changes.
How is the IMO check digit calculated?
Multiply each of the first six digits by 7, 6, 5, 4, 3, 2 respectively, sum the products, and take the result modulo 10. That single digit must equal the seventh digit of the IMO number.
Is the IMO number the same as MMSI?
No. IMO number identifies the hull for its lifetime. MMSI is a 9-digit radio identifier tied to the vessel's AIS transponder and changes when the vessel re-flags.